1. Field of the Invention
The present invention relates to electromechanical machines and, more particularly, relates to a system for maintaining superconducting coils of a rotor at a desired temperature.
2. Discussion of the Related Art
Electromechanical machines such as generators and motors include rotor and stator windings that create a magnetic field to rotate the rotor. Synchronous motors are well known in the art as comprising a rotor that rotates as a result of magnetic flux created between an armature winding and a stator winding. Synchronous motors having superconducting rotor coils are unique in that the coils operate without any resistance to electrical current. As a result, higher current densities may be achieved that are not possible in conventional conductors which, in turn, allows for stronger magnetic fields in electromechanical machines. These motors therefore have a notably higher efficiency than conventional motors.
Presently, low temperature superconducting coils are known to operate below approximately 10xc2x0 K., and high temperature superconducting coils are known to operate above approximately 30xc2x0 K. If the operating temperature rises significantly above the normal operating temperature for the superconductor, the coil will act as a conventional conductor, and electric losses will occur within the rotor. It is therefore important to design a coolant system that maintains the superconducting coil at its designed temperature.
In present high temperature superconducting devices, a cryogenic rotary transfer coupling delivers a coolant from a stationary cryogenic cooler to the rotor, thereby cooling the rotor coils, and returns warm coolant to the cooler. Because portions of the coupling rotate during operation, and other portions are stationary, a relative motion gap is formed between the rotating and stationary parts. This gap is a significant path for parasitic heat leakage into the coupling. Present relative motion gap arrangements can result in warmer flow than necessary returning to the cryogenic cooler, thereby decreasing the efficiency of the coupling. Additionally, the complex physical orientation of the annular gap adds cost and complexity to the rotor assembly during manufacturing, and requires tighter tolerances of the machined parts.
Additionally, in cooling systems that use a liquid helium supply flow and a gaseous helium return flow, a large temperature differential results between the two flows. As a result, significant conduction may occur between the outer walls of the supply tube and the outer walls of the return tube, thereby decreasing the efficiency of the overall cooling system.
The need has therefore arisen to provide a cooling system for a superconducting rotor that does not incorporate the difficulties in manufacturing and efficiency associated with prior art cooling systems.
It is therefore a first object of the present invention to provide a cryogenic transfer coupling within a superconducting rotor having an annulus that permits the transfer coupling to be manufactured by a simple and cost-effective manufacturing process.
It is a second object of the invention to provide a cryogenic transfer coupling within a superconducting rotor having a relative motion gap that does not materially adversely affect the efficiency of the coupling.
It is a third object of the invention to manufacture a cooling system incorporating the above two objects that further permits the temperature of the return flow to be either slightly greater or significantly greater than the temperature of the supply flow.
In accordance with a first aspect of the invention, a cryogenic rotary transfer coupling is provided for delivering a cryogenic coolant, such as gaseous helium, from a cryogenic cooler to a supply flow path that extends axially through a rotor shaft, thereby permitting the coolant to enter the rotor and cool the superconductive coils. The coolant then returns to the cooler via a return flow path. Both the supply and the return flow paths have stationary parts connected to the cooler, and rotating parts extending into the rotor. A relative motion gap is therefore formed between the stationary and rotating parts of the coupling. Both stationary and rotating walls of the gap provide a solid conduction path from the ambient environment to the cold part of the coupling. Also the coolant filling the gap may contribute to heat leakage to the cold part via convection. In order to minimize this parasitic heat leakage, at least a portion of the gap is a narrow and long annulus bounded by two concentric thin wall tubes. The tube axis coincides with an axis of rotation, and both tubes are vacuum insulated. Depending on the coupling design, the relative motion gap may be continuous or it may comprise a plurality of segments. The coupling is designed such that every straight line extended from the rotor axis radially outwardly and perpendicularly to the axis will cross the relative motion gap no more than once. This design of the relative motion gap reduces the need for inserts and spacers that are necessary with couplings having other types of relative motion gaps.
In accordance with a second aspect of the invention, a plurality of vacuum cavities exists within the coupling, thereby eliminating heat transfer that would otherwise increase the temperature of the supply and return flows. Additionally, the temperature gradient within the gap is such that the temperature of the fluid flowing through the gap to be returned to the cooler is not increased significantly by the parasitic heat leak. The relatively cool return flow thereby reduces the energy needed to sufficiently cool the return flow and increasing the overall efficiency of the system.
In accordance with a third aspect of the invention, one embodiment is designed to accommodate cooling systems having a return flow of only a few degrees greater than the supply flow, thereby minimizing the concern for heat loss due to conduction between the return flow and the supply flow. In a second embodiment, the potential for conductive heat loss between the return flow and the supply flow is minimized, thereby accommodating cooling systems having return flow temperatures that are significantly greater than the supply flow temperature.